Corrosion is an electrochemical process involving an anode, an electrolyte, and a cathode. When a piece of metal corrodes, the electrolyte provides charged hydrogen ions and hydroxide molecules to the metal. Corrosion occurs as the charged hydroxide ions combine with the metal, metallic hydroxides are created and metal is liberated into the electrolyte. Electrons are released into the metal by this reaction. A balancing cathodic reaction also occurs when the hydrogen ions flow through the electrolyte to the cathode, electrons are released from the metal and hydrogen gas is formed. These reactions involve a transfer of charge and therefore the sum of the electrons released by the corrosion of steel and aluminum (anodic reactions) must be consumed by the hydrogen evolution (cathodic reaction).
A cathodic protection system is implemented in water heaters to prevent corrosion of the water heater tank. The cathodic protection system includes an anode rod, which is electrically connected to the metal water heater tank. The anode rod is comprised of a metal, such as aluminum, magnesium, zinc, or other alloy that is more active than the metal tank of the water heater. The water heater tank is generally comprised of glass coated steel.
When water is introduced into the water heater tank, a galvanic circuit is created between the metal tank (and/or connectors) and the anode rod. As a result, electrical current flows from the anode, to the cathode, and ions flow through the water to complete the circuit between the anode and cathode, thus, the anode rod begins to corrode. The water supply may be modeled as a resistor within the electrical circuit. If the water supply has a high mineral content, the resistance through the water is low and current flow will increase, resulting in a corresponding increase in the consumption of the anode rod.
The addition of a resistor in the galvanic circuit can reduce the consumption time of the anode. Current resistored anodes, such as disclosed in U.S. Pat. Nos. 5,256,267 and 5,334,299 issued to Roden, are relatively difficult to assemble and the assembly is fragile. The exposed solder connection of the first lead of the resistor to the metal cap is susceptible to damage during installation in a water heater and during shipping and handling of the water heater and/or anode assembly. A fragile resistor connection can be broken, and can result in a loss of the connection of the anode to the water heater tank, resulting in accelerated corrosion failure of the water heater tank.